Understanding tire sizes and ratings

"Visually, the ML430 will be distinguished from its V6 powered sibling with body-colored bumpers, rocker panels and side moldings and performance-oriented 17-inch alloy wheels with super-large 275/55R-17 M+S rated tires."

As you see, MBUSI claimed that the 17 inch tires of the ML430 are super-large (compared to the 255/65R16 tire of the ML320)

In fact the "super-large" 17 inch tire is slightly smaller than the 16" !

Here is why: Whether it is a 16", 17" or 18" tire, does not say anything about the diameter of a tire. 16, 17 or 18 indicates only the size of the rim the tire will be mounted on.

Only the 255/65 portion of the 255/65R16 tire indicates roughly the size (diameter).

255/65 R 16

65 % of

255 mm

= 165.75 mm (6.53")

2 x 6.53" + 16"

= 29.06"

275/55 R 17

55% of

275 mm

= 151.25 mm (5.95")

2 x 5.95" + 17"

= 28.90"

However, the "General Tire" 255/65 R 16 found on the ML320 has in fact a diameter of 28.21 inches and not 29.06 inches as calculated above.

That is almost one inch off. Why?

Truth is, that almost every manunufacturer makes a 255/65 R 16 tire at a different diameter. So when replacing a bad tire, make sure it is not only the same size (like 255/65 R 16) but it is also by the same manufacturer.